This invention relates to processes for the conversion of carbonaceous solid materials such as coal to clean and valuable hydrocarbon liquids and gases. More specificially, this invention relates to a process for liquefying coal to produce a liquid fuel which is substantially free of the mineral components normally found in coal as well as undissolved coal particles.
It has long been known that hydrocarbon gases, liquids, pitch and/or chemicals can be obtained from coal, which is mined in vast quantities in many parts of the world. Heretofore, the prior art has employed generally destructive distillation processes for the conversion of coal to the desired products. More recently, the art has developed high pressure hydrogenation and solvent extraction techniques to effect the conversion. The solvent extraction technique in which the coal is contacted with a selective solvent and subsequently hydrogenated and distilled appears particularly promising but the procedures thus far adapted all suffer from the problem of removing the mineral matter always associated with coal, commonly referred to as "ash", as well as finely divided, undissolved coal. It has been found that if this contaminating mineral matter is not removed from the extract, considerable processing difficulties occur in subsequent treating or recovery procedures, particularly in the subsequent hydrogenation processes which utilize a catalyst. The mineral matter tends to deposit on the catalyst causing rapid deactivation or poisoning of the catalyst.
Numerous techniques for removing the mineral matter and undissolved coal have been suggested including filtration, hydroclones, centrifuging, distillation, fluid coking, settling, water washing, electrostatic precipitation, solvent precipitation, and foam flotation, and the patent literature describes many of these techniques in detail. With the exception of vacuum distillation all of these methods are relatively ineffectual for the removal of particles in the submicron range. Even with distillation the erupting surface of the coal solution permits suspended small particles to be carried over with the distillate. Solvent precipitation, which is the subject of many patents of which U.S. Pat. No. 3,791,956 issued Feb. 12, 1974 to Gorin et al is illustrative, is relatively expensive to perform and rather slow, usually requiring many hours of centrifuging or several days of gravity settling. Generally solvent precipitation techniques are based upon the use of a precipitating solvent, sometimes called an antisolvent, to effect agglomeration of the finely divided solids by precipitating asphaltenes, contained within the liquefied coal slurry, onto the surface of the finely divided solids causing them to stick together and form small, loosely structured agglomerates which enhance the settling rate. Asphaltenes, which are defined as benzene soluble and n-pentane insoluble materials in coal extracts, are barely soluble in the coal solvent and insoluble in the short chain organic precipitating solvent, and they are precipitated because of the high mutual solubility of the short chain solvents with the coal solvent and some solution components. As a portion of the asphaltenes, which make up the high boiling point fraction of solubilized coal and can range as high as 40% by weight of the solubilized coal, end up in the tailings the yield of solubilized coal is considerably reduced unless efforts are made to recover them. Solvent extraction with solvents such as benzene or tetrahydrofuran is effectively the only practical method and this has the disadvantage that it frees the individual solid particles again and the problem of separation again occurs.